The present invention relates to semiconductor nanocrystals.
Semiconductor nanocrystals are very small crystallites of semiconductor material, also known as quantum dots, which have the opto-electronic properties of semiconductors. They are typically prepared as colloids, and as such, display physico-chemical properties of molecules.
A striking feature of semiconductor nanocrystals is that their color may be controlled by their size. This is a direct consequence of quantum confinement on the electronic states, giving these nanocrystals great potential in systems, such as light emitting diodes, photovoltaic cells and future nanoelectronic devices. For these and other applications, it would be desirable to control the electron occupation of the nanocrystals, in the same manner as n- and p-type doping is used to control the electron occupation in bulk semiconductors.
Attempts to dope semiconductor nanocrystals by introducing impurity atoms, as is typically done with bulk semiconductors, have been unsuccessful. Impurities tend to be expelled from the small crystalline cores. Furthermore, thermal ionization of the impurities to provide free carriers in the nanocrystals is hindered by the strong confinement that results from the small size of the nanocrystals. Finally, some attempts at doping have resulted in charge being trapped in surface states, not in the quantum-confined states of the nanocrystals.